Pretty sure the only thing it takes into account is if you have visited the dungeon entrance, then your iLev gear. I remember reading that it would favor dungeons you have not finished, but in LK, I found that to be bunk. I would get just random dungeons that I had all the achievements just as often as those as I did not.

Also random is not as random as you may think. If you are not using your own 5 man group. The extra 'random people' you get my have specifically chosen that dungeon for a drop or some other reason and therefore it is not random at all. Like always getting crusade tournament heroic because the tanks were going for tank trinkets etc.